The present invention is concerned with degradable polymers and the production of materials therefrom. These polymers and materials find utility in polymer therapeutics and pharmaceutical compositions for the treatment of disease.
Polymer Therapeutics (Duncan R., Polymer therapeutics for tumour specific delivery Chem and Ind 1997, 7, 262-264) are developed for biomedical applications requiring physiologically soluble polymers and include biologically active polymers, polymer-drug conjugates, polymer-protein conjugates, and other covalent constructs of bioactive molecules. An exemplary class of a polymer-drug conjugate is derived from copolymers of hydroxypropyl methacrylamide (HPMA) which have been extensively studied for the conjugation of cytotoxic drugs for cancer chemotherapy (Duncan R: Drug-polymer conjugates: potential for improved chemotherapy. Anti-Cancer Drugs, 1992, 3, 175-210. Putnam D, Kopecek J: Polymer conjugates with anticancer activity. Adv. Polym. Sci., 1995, 122, 55-123. Duncan R, Dimitrijevic S, Evagorou E: The role of polymer conjugates in the diagnosis and treatment of cancer. STP Pharma, 1996, 6, 237-263). An HPMA copolymer conjugated to doxorubicin known as PK-1, is currently in Phase II evaluation in the UK. PK-1 displayed reduced toxicity compared to free doxorubicin in the Phase I studies (Vasey P, Twelves C, Kaye S, Wilson P, Morrison R, Duncan R, Thomson A, Hilditch T, Murray T, Burtles S, Cassidy J: Phase I clinical and pharmacokinetic study of PKI (HPMA copolymer doxorubicin): first member of a new class of chemotherapeutic agents: drug-polymer conjugates. Clin. Cancer Res., 1999, 5, 83-94). The maximum tolerated dose of PK-1 was 320 mg/m2 which is 4-5 times higher than the usual clinical dose of free doxorubicin.
The polymers used to develop Polymer Therapeutics may also be separately developed for other biomedical applications where the polymer can form aggregates such as polymeric micelles and complexes. Another important set of medical applications include those that require the polymer be used as a material, rather than as a physiologically soluble molecule. Thus, drug release matrices (including microspheres and nanoparticles), hydrogels (including injectable gels and viscious solutions) and hybrid systems (e.g. liposomes with conjugated poly(ethylene glycol) (PEG) on the outer surface) and devices (including rods, pellets, capsules, films, gels) can be fabricated for tissue or site specific drug delivery. Polymers are also clinically widely used as excipients in drug formulation. Within these three broad application areas: (1) physiologically soluble molecules, (2) materials and (3) excipients, biomedical polymers provide a broad technology platform for optimising the efficacy of an active therapeutic drug.
Covalent conjugation of a drug to a soluble, biocompatible polymer can result in improved efficacy of the drug. Compared to the free, unconjugated drug, polymer-drug conjugates exhibit this improvement for the following main reasons: (1) altered biodistribution, (2) prolonged circulation, (3) release of the drug in the proteolytic and acidic environment of the secondary lysosome after cellular uptake of the conjugate by pinocytosis and (4) more favourable physicochemical properties imparted to the drug due to the characteristics of large molecules (e.g. increased drug solubility in biological fluids).
For the treatment of cancer there are marked improvements in therapeutic efficacy and site specific passive capture through the enhanced permeability and retention (EPR) effect. The EPR effect results from enhanced permeability of macromolecules or small particles within the tumour neovasculature due to leakiness of its discontinuous endothelium. In addition to the tumour angiogenesis (hypervasculature) and irregular and incompleteness of vascular networks, the attendant lack of lymphatic drainage promotes accumulation of macromolecules that extravasate. This effect is observed in many solid tumours for macromolecular agents and lipids. The enhanced vascular permeability will support the great demand of nutrients and oxygen for the rapid growth of the tumour. Unless specifically addressed for tumour cell uptake by receptor-medicated endocytosis, polymers entering the intratumoural environment are taken up relatively slowly by fluid-phase pinocytosis.
An increasing number of physiologically soluble polymers have been used as macromolecular partners for the conjugation of bioactive molecules. Many polymers have the disadvantage of being non-degradable in the polymer mainchain. For example, PEG (Monfardini C, Veronese F: Stabilization of substances in circulation. Bioconjugate Chem., 1998, 9, 418-450. Zalipsky S: Chemistry of polyethylene glycol conjugates with biologically active molecules. Adv. Drug Delivery Rev., 1995, 16, 157-182. Delgado C, Francis G, Fisher D: The uses and properties of PEG-liked proteins. Crit. Rev. Ther. Drug Carrier Syst., 1992, 9, 249-304. Nucci M L, Shorr D, Abuchowski A: The therapeutic values of poly(ethylene glycol)-modified proteins. Adv. Drug Delivery Rev., 1991, 6, 133-151. Nathan A, Zalipsky S, Ertel S, Agathos S, Yarmush M, Kohn J: Copolymers of lysine and polyethylene glycol: A new family of functionalized drug carriers. Bioconjugate Chem., 1993, 4, 54-62) and HPMA (Putnam D, Kopecek J: Polymer conjugates with anticancer activity. Adv Polym. Sci., 1995, 122, 55-123. Duncan R, Dimitrijevic S, Evagorou E: The role of polymer conjugates in the diagnosis and treatment of cancer. STP Pharma, 1996, 6, 237-263) copolymers have been extensively studied for conjugation. PEG is also generally used in the pharmaceutical industry as a formulation excipient. These hydrophilic polymers are soluble in physiological media, but their main disadvantage is that the polymer mainchain does not degrade in vivo. Thus it is not possible to prohibit accumulation of these polymers in the body. Only polymers with a molecular weight lower than the renal threshold can be used for systemic administration. It is imperative that for the systemic use of non-degradable polymers such as HPMA and PEG only molecules of a molecular weight which are readily cleared be administered or else long-term deleterious accumulation in healthy tissue will invariably result (Seymour L, Duncan R, Strohalm J, Kopecek J: Effect of molecular weight (Mw) of N-(2-hydroxypropyl)methacrylamide copolymers on body distributions and rate of excretion after subcutaneous, intraperitoneal and intravenous administration to rats. J. Biomed. Mater. Res., 1987, 21, 1341-1358. Schneider P, Korolenko T, Busch U: A review of drug-induced lysosomal disorders of the liver in man and laboratory animals. Microscopy Res. Tech., 1997, 36, 253-275. Hall C, Hall O: Experimental hypertension elicited by injections of methyl cellulose. Experientia, 1961, 17, 544-454. Hall C, Hall O: Macromolecular hypertension: hypertensive cardiovascular disease from subcutaneously administered polyvinyl alcohol. Experientia, 1962, 18, 38-40).
Although some natural polymers such as polysaccharides have the advantage of being degradable in vivo, e.g. dextran, they typically lack a strict structural uniformity and have the propensity upon chemical modification (i.e. conjugation of a bioactive molecule) to become immunogenic or non-degradable (Vercauteren J, Bruneel D, Schacht E, Duncan R: Effect of the chemical modification of dextran on the degradation by dextranases. J. Bio. Comp. Polymers, 1990, 5, 4-15. Shalaby W, Park K: Chemical modification of proteins and polysaccharides and its effect on enzyme-catalysed degradation. In: Shalaby S, ed. Biomedical Polymers. Designed-to-degrade systems. New York: Hanser Publishers, 1994). Other polysaccharides which have been investigated for biomedical conjugation applications include chitosan (Ohya Y, Huang T, Ouchi T, Hasegawa K, Tamura J, Kadowaki K, Matsumoto T, Suzuki S: a-1,4-Polygalactosamine immobilised 5-fluorouracils through hexamethylene spacer groups via urea bonds. J. Cont. Rel., 1991, 17, 259-266), alginate (Al-Shamkhani A, Duncan R: Synthesis, controlled release properties and antitumour activity of alginate cis-aconityl daunomycin conjugates. Int. J. Pharm., 1995, 122, 107-119. Morgan S, Al-Shamkhani A, Callant D, Schacht E, Woodley J, Duncan R: Alginates as drug carriers: covalent attachment of alginates to therapeutic agents containing primary amine groups. Int. J. Pharm., 1995, 122, 121-128), hyaluronic acid (Schechter B, Neumann A, Wilchek M, Amon R: Soluble polymers as carriers of cisplatinum. J. Cont. Rel., 1989, 10, 75-87), 6-O-carboxymethyl chitan (Ohya Y, Nonomura K, Ouchi T: In vivo and in vitro antitumor activity of CM-Chitin immobilized doxorubicins by lysosomal digestible tetrapeptide spacer groups. J. Bioact. Compat. Polymers, 1995, 10, 223-234) and 6-O-carboxymethyl pullulan (Nogusa H, Yano T, Okuno S, Hamana H, Inoue K: Synthesis of carboxymethylpullulan peptide doxorubicin conjugates and their properties. Chem. Pharm. Bull., 1995, 43, 1931-1936).
Other natural polymers such as proteins can also be used to conjugate a bioactive molecule. For example albumin has been investigated as a protein used to conjugate a bioactive molecule (Balboni P, Minia A, Grossi M, Barbanti-Brodano G, Mattioli A, Flume L: Activity of albumin conjugates of 5-fluorodeoxyuridine and cytosine arabinoside on poxviruses as a lysosomotropic antiviral chemotherapy. Nature, 1976, 264, 181-183. Trouet A, Masquelier M, Baurain R, Campaneere D: A covalent linkage between daunorubicin and proteins that is stable in serum and reversible by lysosomal hydrolases as required for a lysosomotropic drug-carrier conjugate. In vitro and in vivo studies. Proc. Natl. Acad. Sci. USA, 1982, 79, 626-629. Dosio F, Brusa P, Crosasso P, Arpicco S, LCattel: Preparation, characterization and properties in vitro and in vivo of a paclitaxel-albumin conjugate. J. Cont. Rel., 1997, 47(3), 293-304. Yasuzawa T, Tomer K: Structural determination of the conjugate of human serum albumin with a mitomycin C derivative, KW-2149, by matrix assisted laser desorption/ionization mass spectrometry. Bioconjugate Chem., 1997, 8, 391-399. Wunder A, Stehle G, Schrenk H, Hartung G, Heene D, Maier-Borst W, Sinn H: Antitumor activity of methotrexate-albumin conjugates in rats bearing a Walker-256 carcinoma. Int. J. Cancer, 1998, 76, 884-890). The major limitations for using a protein to conjugate a bioactive compound include the propensity for inducing immunogenicity and non-specific degradation of the protein in vivo, and denaturation and irreversible alteration of the protein during preparation of the conjugate. Other proteins such as transferrin, which binds to the tranferrin receptor and thus have the potential to undergo receptor-mediated uptake (Tanaka T, Kaneo Y, Miyashita M: Intracellular disposition and cytotoxicity of transferrin-mitomycin C conjugate in HL60 cells as a receptor-mediated drug targeting system. Biol. Pharm. Bull., 1998, 21(2), 147-152) and various immuno-conjugates (Gaal D, Hudecz F: Low toxicity and high antitumour activity of daunomycin by conjugation to an immunopotential amphoteric branced polypeptide. Eur. J. Cancer, 1998, 34(1), 155-16. Trail P, Willner D, Hellestrom K: Site-directed delivery of anthracyclines for the treatment of cancer. Drug Dev. Res., 1995, 3, 196-209. Eno-Amooquaye E, Searle F, Boden J, harma S, Burke P: Altered biodistribution of an antibodyxe2x80x94enzyme conjugate modified with polyethylene glycol. Br. J. Cancer, 1996, 73, 1323-1327. Flanagan P, Duncan R, Subr V, Ulbrich K, Kopeckova P, Kopecek J: Evaluation antibody-[N-(2-hydroxypropyl)methacrylamide] copolymer conjugates as targetable drug-carriers. 2. Body distribution of anti Thy-1,2 antibody, anti-transferrin receptor antibody B3/25 and transferrin conjugates in DBA2 mice and activity of conjugates containing daunomycin against L1210 leukaemia in vivo. J. Cont. Rel., 1992, 18, 25-38. Springer C, Bagshawe K, Sharma S, Searle F, Boden J, Antoniw P, Burke P, Rogers G, Sherwood R, Melton R: Ablation of human choriocarcinoma xenografts in nude mice by antibody-directed enzyme prodrug therapy (ADEPT) with three novel compounds. Eur. J. Cancer, 1991, 11, 1362-1366.) also have been investigated. Monodisperse molecular weight distribution is often claimed to be a significant advantage for using proteins to conjugate drugs, but this can only be useful if a single species of the protein-drug conjugate can be reproducibly prepared on adequate scale which is stable on storage. This is generally not economically or technologically possible to achieve in practice. Thus, there is a need for degradable synthetic polymers developed for biomedical application, and specifically for conjugation applications, which can address the limitations inherent in the use of natural polymers for these applications.
Synthetic polymers which have been prepared and studied that are potentially degradable include polymers derived from amino acids (e.g. poly(glutamic acid), poly[5N-(2-hydroxyethyl)-L-glutamine), xcex2-poly(2-hydroxyethyl aspartamide), poly(L-glutamic acid) and polylysine). These polymers when prepared for conjugation applications that require physiological solubility do not degrade in vivo to any extent within a time period of 10-100 hours. Additionally polymers and copolymers including pseudo-poly(amino acids) (James K, Kohn J: Pseudo-poly(amino acid)s: Examples for synthetic materials derived from natural metabolites. In: Park K, ed. Controlled Drug Delivery: Challenges and Strategies. Washington, D.C.: American Chemical Society, 1997; 389-403) and polyesters such as copolymers of polylactic and poly(glycolic acid), poly(xcex1 or xcex2-malic acid) (Abdellaoui K, Boustta M, Vert M, Morjani H, Manfait M: Metabolite-derived artificial polymers designed for drug targeting, cell penetration and bioresorption. Eur. J. Pharm. Sci., 1998, 6, 61-73. Ouchi T, Fujino A, Tanaka K, Banba T: Synthesis and antitumour activity of conjugates of poly (xcex1-malic acid) and 5-fluorouracil bound via ester, amide or carbamoyl bonds. J. Cont. Rel., 1990, 12, 143-153), and block copolymers such as PEG-lysine (Nathan A, Zalipsky S, Ertel S, Agathos S, Yarmush M, Kohn J: Copolymers of lysine and polyethylene glycol: A new family of functionalized drug carriers. Bioconjugate Chem., 1993. 4, 54-62.), poly(lysine citramide) (Abdellaoui K, Boustta M, Vert M, Morjani H, Manfait M: Metabolite-derived artificial polymers designed for drug targeting, cell penetration and bioresorption. Eur. J. Pharm. Sci., 1998, 6, 61-73) and amino acid-PEG derived block copolymers (Kwon G, Kataoka K: Block copolymer micelles as long-circulating drug vehicles. Adv. Drug. Del. Rev., 1995, 16, 295-309. Alakhov V, Kabanov A: Block copolymeric biotransport carriers as versatile vehicles for drug delivery. Exp. Opin. Invest. Drugs, 1998, 7(9), 1453-1473) have also been investigated for conjugation.
The three main parts of a polymer-drug conjugate: (1) polymer, (2) linker and (3) conjugated drug all have defined biological function. Together these components produce a distinct profile of pharmacological, pharmacokinetic and physicochemical properties typical of polymer-drug conjugates. The polymer is not a mere carrier for the pharmacologically active drug. The properties of the polymer are directly responsible for defining the circulation half-life, rate of cellular uptake, minimising toxicity of potent cytotoxic drugs and imparting favourable physicochemical properties (e.g. increasing the solubility of lipophilic drugs).
Lysosomes also contain a vast array of hydrolytic enzymes including proteases, esterases, glycosidases, phosphates and nucleases. Drugs have been conjugated to polymers using conjugation linkers that degrade in the lysosome while remaining intact in the bloodstream. Since many drugs are not pharmacologically active while conjugated to a polymer, this results in drastically reduced toxicity compared to the free drug in circulation.
A wide variety of linkages have been used to covalently bind a drug to the polymeric carrier. Several examples include, amide, ester, hydrazide, urethane, carbonate, imine, hydroxyl, thioether, azo and Cxe2x80x94C.
Following the concept of lysosomotropic drug delivery two broad classes of pendent chain linkers have emerged as the main focus of research over the last two decades. These are:
1. Peptidyl linkers designed to be stable in the bloodstream, but degradable by lysosomal enzymes and thus able to release the drug intracellularly.
2. Acid-labile, pH dependent linkers which are designed to remain stable in plasma at neutral pH (7.4), but release drug intracellularly by hydrolysis in the more acidic environment of the endosome and lysosome (pH 5.5 to 6.5).
Peptide linkers have been shown to mediate lysosomotropic drug delivery (wherein the drug preferentially accumulates in the lysosome). It has become apparent that one of the successful methods of control of the rate and location of drug release from pendent chain polymers has occurred favourably when a drug is bound to the polymer backbone via a peptidyl side-chain.
Since the discovery that peptidyl side chains in HPMA copolymers could be designed for cleavage by model enzymes such as chymotryspin, tryspin and papain recent studies have seen the systematic development of HPMA copolymer-anticancer conjugates. These contain peptidyl linkers tailored for cleavage by lysosomal proteases. Such linkers have now become more widely used in many different polymer conjugates.
Shen and Ryser (Biochim, Biophys. Res. Commun., 1981, 102, 1048-1054) disclose pH-sensitive linkers of n-cis-aconityl and n-maleyl groups used to conjugate daunomycin to amino ethyl polyacrylamide and to poly(d-lysine). Hydrolysis of the cis-aconityl spacer released daunomycin from poly-(d-lysine) in the lysosomes.
Diener, et al (Science, 1986, 231, 148-150) have shown that daunomycin, when conjugated to a targeting antigen by a cis-aconityl spacer, remains inactive in the extracellular system, but becomes active after cleavage within the acidic lysosomal environment of a target cell.
Dilman, et al (Cancer res., 1988, 48, 6097-6102) have conjugated daunorubicin to the anti-T-cell monaclonal antibody T101 using a cis-aconityl group. The pH sensitivity of the linkage was confirmed. A similar study using a monaclonal antibody conjugated to doxorubicin has been shown to suppress the growth of established tumour xenografts in nude mice (Yang and Ricefelt Proc. Natl. Acad. Sci., 1988, 85, 1189-1193).
GB 2,270,920 discloses a therapeutically useful alginate-bioactive agent conjugate, wherein the alginate and bioactive agent are connected covalently via an acid labile linkage, preferably a cis-aconityl group.
An advantage of conjugating a drug via an acid-labile linker is that free drug alone can be released from the pendent chain rather than amino acid or peptide drug derivatives which can occur with peptidyl linkers.
The relatively low pH within the endosomal, and lysosomal compartments and the observation that the extracellular, interstitial environment in some tumours is also acidic, has inspired the development of pendent chain-linkers that hydrolytically degrade more quickly at pH values less then 7.4. Cis-aconityl acid and Schiff base derivatives are the two predominant types of hydrolytically labile linkers that have been explored.
One object of the present invention is to provide pH dependant degradeable polymers.
A further object of the present invention is to provide biocompatible, degradable polymers that will hydrolytically degrade at faster rates at acidic pH values than at neutral pH values.
A further object of the present invention is to provide degradable polymers that degrade in the endosome or lysosome, while enabling conjugation to a lysosomally labile bioactive agent.
One embodiment of the invention provides a polymer comprising: a polymeric backbone comprising at least one unit having the structure (I), 
wherein R-R4 comprise groups selected from the group consisting of H, C1-C12 alkyl, C6-C18 aryl, C7-C18 aralkyl, C6-C18 cycloalkyl or any of the group consisting of C1-C12 alkyl, C6-C18 aryl, C7-C18 aralkyl, C6-C18 cycloalkyl substituted, within the carbon chain or appended thereto, with one or more heteroatoms; R and R2 or R and R4 or R and R1 or R2 and R3 may be joined so that with the carbon atom(s) to which they are attached they together form a saturated, partially unsaturated or unsaturated ring system respectively, may have a pendent group which may incorporate a linker unit, (for example a peptide linkage) or a unit having the structure (I); A comprises a proton donating moiety selected from the group consisting of 
B comprises a hydrolytically labile group and is selected from the group consisting of 
wherein each R5 is individually selected from the group consisting of H, C1-C12 alkyl, C6-C18 aryl, C7-C18 aralkyl, C6-C18 cycloalkyl; wherein groups A and B are in a cis-configuration about bond Caxe2x80x94Cb; m is an integer of 0 to 100, n, p and q are each an integer of 0 or 1; Q comprises 1 or more structures selected from the group consisting of 
wherein R6-R11 are individually selected from the same group as defined for group R above and r is an integer between 1 and 5000, preferably 1 to 10 most preferably 1 to 6.
Caxe2x80x94Cb may be a double bond, in which case p and q are 0 and groups A and B are in a cis-configuration across the double bond. R and R2 or R and R4 or R and R1 or R2 and R3, preferably R and R2, may be joined to one another to form part of a C3-C12 ring system which may have none one or more than one unsaturated bond and may be aromatic. When such a ring system is formed and Caxe2x80x94Cb is not a double bond, A and B are in a cis-configuration about bond Caxe2x80x94Cb. Preferably such a ring system is a C3-C7 ring system. The ring system may incorporate any of the groups defined for R or may include one or more Q groups.
When Caxe2x80x94Cb is a single bond, p and q are 1 and R, R1, R4 and A are selected from sterically bulky groups in such a way as to maintain a cis-configuration of A and B about bond Caxe2x80x94Cb. Preferably Caxe2x80x94Cb is a double bond.
Preferably R-R4 are individually selected from the group consisting of hydrogen, methyl, ethyl, propyl, butyl, pentyl and hexyl and isomers thereof, acyl, alkoxy and acyloxy or mixtures thereof. Most preferably R, R2 and R3 are hydrogen.
Preferably each R5 is individually selected from the group consisting of H, methyl, ethyl, propyl, butyl, pentyl and hexyl, preferably hydrogen.
A preferably comprises a group or a protected carboxylic acid group.
B preferably comprises an amide bond, and is most preferably a group 
wherein R5 has been hereinbefore defined.
Q may comprise more than one or a mixture of the structures defined above. Preferably Q comprises a carbonyl group, xe2x80x94NR12xe2x80x94, xe2x80x94Oxe2x80x94 or xe2x80x94CH2xe2x80x94 group, wherein R12 is selected from the group consisting of hydrogen, C1-6-alkyl, preferably methyl, ethyl, propyl, butyl, pentyl and hexyl and isomers thereof. Preferably R12 is a hydrogen atom.
Most preferably Q comprises a carbonyl functionality or a xe2x80x94CH2xe2x80x94 group, especially a carbonyl functionality.
In a particularly preferred embodiment bond Caxe2x80x94Cb is a double bond, R is hydrogen, R2 and R3 are hydrogen, n is 1, m is 1, p and q are 0, A is a carboxylic acid group, B comprises an amide bond and Q comprises an carbonyl group.
Preferably where the polymer contains more than one (I) moiety, the groups A, B, Q, R-R4, m, n, p and q in each individual moiety are the same.
The other components of the polymeric backbone may be other groups having the structure (I), peptide units or other degradeable polymeric, oligomeric or monomeric units. For example, the polymeric backbone may comprise acrylic polymers, alkylene polymers, urethane polymers, amide polymers, polypeptides, polysaccharides and ester polymers. Preferably the backbone components comprise derivatised polyethyleneglycol or copolymers of hydroxyalkyl(meth)acrylamide, most preferably amine derivatised polyethyleneglycol or hydroxypropylmethacrylamide-methacrylic acid copolymers, or derivatives thereof.
A further embodiment of the present invention provides a polymer comprising a polymeric backbone comprising the structure (II) 
wherein A, B, Q, R-R4, m, n, p and q are as defined above; L is a polymeric, oligomeric or copolymeric bridging group which comprises groups selected from the group consisting of acrylic polymers, alkylene polymers, urethane polymers, polyethylene glycols, polyamides(including polypeptides), polysaccharides and polyesters. a is an integer of 1 to 100000, b and c are integers of 0 to 100000 and s is an integer of 0 to 100; D comprises one or more structures individually selected from the group consisting of, 
wherein R14 and R14xe2x80x2 comprise groups individually selected from the same groups as defined for R or may comprise a structure selected from the group consisting of 
wherein n is an integer of 0-100, R15 is selected from the group consisting of hydrogen and C1-C6 alkyl, R16 to R18 are individually selected from the group consisting of H, C1-C12 alkyl, C1-C12 alkenyl, C6-C18 aryl, C7-C18 aralkyl, C5-C18 cycloalkyl or is selected from the group consisting of C1-C12 alkyl, C1-C12 alkenyl, C6-C18 aryl, C7-C18 aralkyl, C6-C18 cycloalkyl substituted, within the carbon chain or appended thereto, with one or more heteroatoms, a pendent group comprising a linker unit, for example a peptide linkage or a unit having the structure (I) or a leaving group; R13 is selected from the group consisting of H, C1-C12 alkyl, C1-C12 alkenyl, C6-C18 aryl, C7-C18 aralkyl, C5-C18 cycloalkyl or is selected from the group consisting of C1-C12 alkyl, C1-C12 alkenyl, C6-C18 aryl, C7-C18 aralkyl, C6-C18 cycloalkyl substituted, within the carbon chain or appended thereto, with one or more heteroatoms, R13 may contain a linker unit, for example a peptide linkage or a unit having the structure (I).
Preferably L comprises a compound selected from the group comprising derivatised polyethyleneglycol and (hydroxyalkyl(meth)acrylamide-methacrylic acid copolymer or amide or ester derivative thereof, most preferably amine derivatised polyethyleneglycol.
Most preferably L comprises a structure comprising a group selected from the group consisting of 
wherein PEG is polyethyleneglycol, R19-R24 may be a pendent group comprising a cleavable linker unit, and comprise groups individually selected from the same groups as defined for R or may comprise a structure selected from the group consisting of 
wherein n and R16 to R18 have been defined hereinbefore.
s is preferably an integer of 1 to 10.
Where L is a group incorporating one of groups R19 to R24, b is preferably 0.
Preferably at least one of R14 to R24 should incorporate a pendent group. Preferably such a pendent group incorporates a cleavable bond. This would be the case wherein R14 to R24 comprise a cleavable group (I) as hereinbefore defined, or a peptidic bond capable of being cleaved by lysosomal enzymes.
Preferably R16-R18 are H, tosylate, Fmoc, halogen, methyl, ethyl, propyl, butyl, pentyl or isomers thereof.
A pendent group as defined hereinbefore may incorporate a bioactive agent to form a conjugate. Preferably the bioactive agent is an anti-cancer agent, for example doxorubicin, daunomycin, taxol and the like. This permits both cleavage of the linker unit, thus releasing drug to the desired site, and biodegradation of the macromolecular carrier, thus reducing side effects associated with the difficulty of clearing such molecules from the system.
Preferably the molecular weight of L is less than 220 kDa, more preferably less than 100 kDa, most preferably less than 30 kDa. Preferably the polymer has a weight of 500D-400 kDa.
A further embodiment of the invention provides prepolymer comprising the structure (III) 
wherein A, B, Q, R-R4, R13, L, m, n, p and q are as defined herein before; Axe2x80x2, Bxe2x80x2, Qxe2x80x2 R1xe2x80x2-R4xe2x80x2, mxe2x80x2, nxe2x80x2, pxe2x80x2, and qxe2x80x2 are selected from the groups as defined for A, B, Q, R1-R4 m, n, p and q respectively; E and K are selected from the group consisting of hydrogen, a protecting group or an activating group and may be the same or different; z is an integer of 1 to 100, y is an integer of 0 to 10 and x is an integer of 0 to 100.
z is preferably 1, y is preferably 1 or 0, x is preferably 1 or 0. Most preferably x=z. Preferably Bxe2x95x90Bxe2x80x2, Qxe2x95x90Qxe2x80x2, Axe2x95x90Axe2x80x2, R1-R4xe2x95x90R1xe2x80x2-R4xe2x80x2, m=mxe2x80x2, n=nxe2x80x2, p=pxe2x80x2 and q=qxe2x80x2. Preferably when B and/or Bxe2x80x2 comprise a carboxylic acid group, E and K are an activating group selected from the group consisting of N-succinimidyl, pentachlorophenyl, pentaflourophenyl, para-nitrophenyl, dinitrophenyl, N-phthalimido, N-norbornyl, cyanomethyl, pyridyl, trichlorotriazine, 5-chloroquinolino. These groups are formed from the reaction with group B, with the following compounds N-hydroxysuccinimide, pentachlorophenyl, pentaflourophenyl, para-nitrophenyl, dinitrophenyl, N-hydroxyphthalimide, N-hydroxynorbornene, cyanomethyl, hydroxypyridine, trichlorotriazine, 5-chloro-8-hydroxy-quinoline respectively. In this embodiment, groups E and K are known as an xe2x80x9cactive estersxe2x80x9d. Preferably E and K are N-succinimidyl. There are other activating moieties that can act as an acylation reagent, such as the mixed anhydrides.
A further embodiment of the present invention provides a prepolymer comprising the structure (IV) 
wherein A, B, Q, R-R4, D, m, n, p and q are as defined above, G and M are selected from the group consisting of hydrogen, an activating group or a protecting group and may be the same or different, i and j are integers of 1 to 10.
i is preferably 1 and j is preferably 1.
Preferably when B and/or D comprise a carboxylic acid group, G and M are an activating group as defined above. Preferably G and M are hydrogen or N-succinimidyl.
A further embodiment provides process for preparing a polymer, copolymer or prepolymer comprising reacting at least one compound having the structure (V) 
wherein R25, R26 and R27 are selected from the group as defined for R; Qxe2x80x3 is selected from the group consisting of carboxylic acid, primary or secondary amine and carbonyl; u is an integer of 0 or 1, v is an integer of 1 to 100, R27 and R25 may be attached to form part of a C3-C12 ring system which may have more than one unsaturated bond and may be aromatic; with at least one compound selected from the group consisting of J and R13LNHR28, wherein L and R13 groups are as defined above and R28 is selected from the same group as defined for R and may be the same or different, J is a compound having at least one primary or secondary amine and a carboxylic acid group and a pendent group incorporating a cleavable bond.
Preferably Qxe2x80x3 is a carboxylic acid group, R27 is hydrogen, u and v are 1, R25 and R26 are hydrogen or methyl. Most preferably R13LNHR28 comprises a NHR29 group, wherein R29 is individually selected from the same group as defined for R28.
A further embodiment provides a method of selectively degrading a polymer comprising the steps of:
a) introducing a polymer as defined by structure (I) or (II) to an environment having a pH of less than 6.5,
b) cleaving said polymer.
A further embodiment provides a method for releasing a bioactive agent comprising the steps of
a) introducing a conjugate as described hereinbefore to an environment having a pH of less than 6.5,
c) cleaving the bioactive agent from the linker group by acid or enzymic hydrolysis,
d) optionally additionally cleaving the polymer by acid or enzymic hydrolysis.
The present invention also comprises compositions which comprise at least one polymer or polymer-bioactive agent conjugate and a carrier. In the case of in vivo treatment it is envisaged that the compositions may be administered orally, by injection, or topically and may comprise a pharmaceutically acceptable excipient.
A further embodiment of the invention includes the use of the novel polymer as a pharmaceutical excipient. As it degrades very quickly at low pH ranges it has application as an excipient for drug formulations prepared for oral administration (i.e. for rapid degradation in the gut or gastro intestinal tract where there are regions of very low pH).
The novel polymers of the present invention may be water soluble or insoluble depending on size and the nature of its components. The degradation products of the polymer are preferably soluble.
In one embodiment, the present invention provides a polymer comprising an acid labile, pH dependent backbone incorporating a cis-aconityl group therein, more specifically a group having the structure (VI). This group is designed to remain stable in plasma at neutral pH (xcx9c7.4), but degrade intracellularly by hydrolysis in the more acidic environment of the endosome or lysosome (xcx9cpH 5.5-6.5). 
Preferably the group (VI) is incorporated into a polymer backbone comprising a polymeric, oligomeric or copolymeric group which comprises functionalised or unfunctionalised polyethyleneglycol, ethyleneglycol copolymers, poly(hydroxyalkyl(meth)acrylamide), for instance hydroxypropylmethacrylamide-methacrylic acid copolymer (or amide or ester derivative thereof) and copolymers of styrene and maleic anhydride, polyurethanes, polyalkylenes and polyamides or amino acid residues. In a particularly preferred embodiment the polymeric backbone should incorporate a functionalised polyethyleneglycol (PEG) polymer or copolymer most preferably an amine functionalised PEG polymer.
The molecular weight of the polymer of the present invention is in the range of 30-400 kDa, while the weight of the prepolymer (III) is preferably less than about 220 kDa in order to ensure that the degraded polymer subunits are cleared from the lysosome and the kidney glomerulus. Most preferably the polymer degradation products have a molecular weight in the range of 0.5 kDa-30 kDa.
One preferred polymer of the present invention is a water soluble polyamide having the formula 3 and is made by the general reaction scheme summarised below: 
wherein PEG is a polyethylene glycol group having a molecular weight in the range 500 Da-100 kDa or derivative thereof and u is an integer in the range of 1-10000.
As shown above, the preferred polymer may be prepared by a 2 step, and optionally 3 step process. In the first step an equivalent of cis-aconitic anhydride, 1, is reacted with a compound containing two primary or secondary amine groups.
Suitable solvents include non-protic solvents including acetonitrile, dimethylformamide, dimethylsulphoxide, DMA, tetrahydrofuran, ethyl acetate, dioxane, acetone etc. Preferably acetonitrile is used. The product is isolated by a suitable method such as solvent separation and the resultant macromonomer 2 is then used as a prepolymer for the production of the acid labile polymer backbone.
Macromonomer (2) may be reacted with two equivalents of an activating group as described hereinbefore (N-hydroxysuccinimide shown) to produce an active monomer. The reason for this is that the unprotected carboxylic acid moieties would otherwise compete in the polymerization reaction, resulting in potential incomplete degradation of the polymer backbone. This situation could, however, be used in the production or enablement of cross-linking and gel formation. If protection is carried out as shown, compound 3 is produced. This compound (3) or compound 2 may then be reacted further with a compound R13LNHR28 as defined hereinbefore. In the diagram shown, R13LNHR28 is simply a amine-difunctionalised PEG molecule. Other compounds that are suitable for use as R13LNHR28 are 
wherein R19-R24 have been defined hereinbefore. Preferably the above defined R14-R19 groups contain a group that is capable of conjugation to a drug, or a precursor thereof, for example, the group R19-R24 should preferably contain a primary or secondary amine.
Suitable methods of attaching a linker molecule or a drug to the polymer backbone are as follows: 
Wherein x is a leaving group such as tosylate, Br and the like.
The reaction of compound 2 or 3 results in one of the preferred polymers of the present invention, compound 4.
The conditions for the step to the final product 4 of the reaction are different than the first, and involve the use of a condensation or coupling reagent type of compound such as a carbodiimide (e.g. dicyclohexyl carbodiimide, diisopropylcarbodiimide, 1-(3-dimethylaminopropyl)-3-ethylcarbodiimide, mixed anhydride reagents (e.g. 2-ethoxy-1-ethoxycarbonyl-1-1,2-dihydroquinoline, 2-isobutoxy-1-isobutoxycarbonyl-2,2-dihydroquinoline, isobutyl chloroformate), phosphonium salts (e.g. benzotriazole-1-yl-oxy-tris-(dimethylamino)-phosphoniumhexafluorophosphate (Castro""s reagent), bromo-tris-pyrrolidino-phosphonium hexafluorophospate, benzotriazole-1-yl-oxy-tris-pyrrolidino-phosphonium hexafluorophosphate), uronium salts (e.g. 2-(1H-benzotriazole-1-yl)-1,2,3,3,-tetramethyluronium hexafluorophosphate, 2-(1H-benzotriazole-1-yl)-1,1,3,3,-tetramethyluronium tetrafluoroborate) and carbonates (e.g. 1,1xe2x80x2-carbonyl-diimidazole, N,Nxe2x80x2-disuccininimidyl carbonate).
The particularly preferred solvents and conditions for this reaction are that molecule 2 is allowed to react in acetonitrile (with DIPC and hydroxysuccinimide) to give the macromonomer 3. The macromonomer 3 is isolated then allowed to react in aqueous carbonate (Na2CO3) at pH 9, 24 h at ambient temperature to give the polymer such as 4.
Another particularly preferred embodiment of the present invention is the production of the water soluble polyamide having the formula 7 and is made by the general reaction scheme summarised below: 
wherein PEG is a polyethylene glycol group having a molecular weight in the range 500 Da-100 kDa or derivative thereof, and v is an integer in the range of 1-10000. As with compound 4, the preferred polymer may be prepared by a 2 step, and option ally 3 step process. In the first step an equivalent of cis-aconitic anhydride, 1, is reacted with a compound containing an amine group and a carboxylic acid group (8) wherein R33 is selected from the same group of compounds as defined for R19-R24.
Suitable solvents again include non-protic solvents, preferably acetonitrile. Macromonomer (2) may be reacted with two equivalents of a protecting group (N-hydroxysuccinimide shown) to produce an active monomer. If protection is carried out as shown, compound 6 is produced. This compound (6) or compound 5 may then be reacted further with a compound R13LNHR28 as defined hereinbefore. In the diagram shown, R13LNHR28 is simply a amine-difunctionalised PEG molecule. Other compounds that are envisaged for use as R13LNHR28 are as shown above.
Another embodiment of the present invention is the production of the water soluble polyamide having the formula 11 and is made by the general reaction scheme summarised below: 
wherein n is an integer of 1-10000. X is a halogen, preferably bromine and PEG is polyethyleneglycol. Dimethylanhydride is reacted with a suitable halogenating agent to produce a halogenated dimethylanhydride. Diamino-PEG is reacted with the halogenated anhydride to produce the polymer.
Suitable solvents for this method again include non-protic solvents, preferably dichloromethane. Since there is the free carboxylate (C-4) there is an equilibrium with the zwitterionic structure 12.
A particular example of this reaction is shown below. 
N-bromosuccinimide is used as the brominating agent. Since there is the free carboxylate (C-4) there is an equilibrium with the zwitterionic structure 12.